1. Field of the Invention
This invention relates to a novel process for producing a cobalt-containing magnetic iron oxide powder having an excellent stability of coercivity under aging and also showing excellent magnetic properties such as squareness, orientability and switching field distribution when the powder is used for a magnetic tape.
2. Description of the Prior Art
Because of high coercivity, the magnetic iron oxide powder coated with a cobalt compound has been widely utilized in the field of magnetic recording such as video and audio recording. However, even higher density of the magnetic recording media is in great demand, and further improvements of the properties of such recording media are required.
In order to satisfy these demands, various methods for producing cobalt-containing and ferrous magnetic iron oxide powders have lately been proposed. Among such methods, the following are typical: (1) A cobalt salt solution is added to a water suspension of .gamma.-Fe.sub.2 O.sub.3, then an alkali is added thereto to coat the particle surfaces of .gamma.-Fe.sub.2 O.sub.3 with a cobalt hydroxide and then an aqueous solution of a ferrous salt is added to said suspension (Japanese Patent Kokoku (Post-Exam. Publn.) No. 48444/81); (2) The method (1) above is carried out under heating (Japanese Patent Kokai (Laid-Open) No. 104721/81); and (3) The .gamma.-Fe.sub.2 O.sub.3 particles are dispersed in an aqueous mixed solution of a ferrous salt and a cobalt salt and then an alkali is added thereto to coat the particle surfaces of .gamma.-Fe.sub.2 O.sub.3 with ferrous hydroxide and cobalt hydroxide simultaneously (Japanese Patent Kokoku (Post Exam. Publn.) No. 36751/77).
However, the magnetic iron oxide powder obtained according to the above-mentioned method (1) or (2) is unsatisfactory in stability of coercivity under aging, and the magnetic tape made by using such powder is found unsatisfactory not only in stability of coercivity under aging but also in other magnetic properties such as squareness, orientability and switching field distribution. Also, according to the method (3) mentioned above, the epitaxial reaction advances rapidly to invite some serious defects such as reduced stability of coercivity under aging and the broadened distribution of coercivity. Thus, the methods hitherto proposed have room for further improvements.